The present invention relates to a process for casting a metallic semi-finished product into a casting.
Castings, in particular light-metal castings, are often reinforced by inserts which are integrated into the component by being cast in. These inserts form local improvements to the material, for example an improved resistance to wear, a higher mechanical strength or thermal stability.
The technical difficulty involved in incorporating inserts of this type by casting often resides in the unsatisfactory adhesion between the relatively smooth surface of the insert and the solidified casting metal. The brief contact time between the metallic melt and the insert and the poor wetting which exists in various material pairings in most cases prevents chemical bonding or alloying between the casting metal and the insert at their adjoining surfaces. A gap at the adjoining surface may impair heat transfer or form a mechanical weak point.
This problem is currently combated by roughening the surface of the insert. The roughening is sometimes carried out by sand-blasting or, as described in German Published Patent Application No. 197 50 687 with reference to the example of a cylinder liner, by material-removing machining of the surface followed by sand-blasting. As a result, microscopic undercuts are produced at the surface of the semi-finished product and lead to a clamping fit between the casting metal and the insert.
However, pure sand-blasting does not lead to desired undercuts, but rather substantially leads to recesses on the surface. The process described in German Published Patent Application No. 197 50 687 does produce the desired undercuts, but is overall very expensive.
It is an object of the present invention to improve the bonding of inserts in accordance with the prior art and of making this process less expensive.
The above and other beneficial objects of the present invention are achieved by providing a process as described herein.
The process according to the present invention includes an insert being roughened by high-pressure water blasting before it is inserted into a casting mold. The high-pressure water blasting cleans the surface and produces undercuts on the surface of the insert, leading to, e.g., good securing of the casting metal after it has solidified.
The insert may be a metallic insert. Under certain conditions and with certain surface properties, it is also possible to use intermetallic components, hard metal, metal-ceramic composites, metal matrix composites (MMC), intermetallic ceramic composites, ceramics, natural inorganic materials, etc. as inserts.
It has been found that the optimum pressure range for a water jet may be between 1500 bar and 2000 bar. Above 2000 bar the surface may be damaged excessively, while below 1500 bar the formation of undercuts may be insufficient. Moreover, for these relatively low pressures it is possible to use pumps of existing pressure installations, such as, for example, for component deburring, with the result that the investment costs may be reduced.
The roughening of the surface may be performed by one or more fan-jet nozzles with an elliptical opening. Unlike full-jet nozzles with a circular opening, fan-jet nozzles provide a surface jet which widens and a groove-free surface. The use of a plurality of nozzles at the same time may reduce the machining time.
To achieve a particularly groove-free surface, an outlet angle of the high-pressure water jet may be between 20 and 34xc2x0.
An example manner of roughening the surface economically and quickly may be to use an array of nozzles, which may be arranged perpendicular to the surface.
Hypereutectic aluminum-silicon alloys may be particularly suitable for roughening the surface using the process according to the present invention. At the surface, semi-finished products made from materials of this type may have hard silicon-rich phases and relatively soft aluminum-rich phases. The hard, silicon-rich phases are torn out by the water-blasting and leave behind the desired undercuts at the surface.
The following explains the process according to the present invention in more detail with reference to an example and two figures.